Fuel burning method



Feb. 1a, 1941.

P. R. GROSSMAN FUEL BURNING iamon mm: .Jan. 29', 1942 Fig. 1

3 Sheets-Sheet 1.

INVENTOR.

BY F ul 1?. Gmssman ATTORNEY- Patented Feb. 18, 1947 UNITED STATESPATENT OFFICE FUEL BURNING IHETHOD Paul R. Grossman, Cleveland, Ohio,assignor. to z The Babcock & Wilcox Company, Newark, N. J., acorporation of NewJersey Application January 29, 1942,Serlal.No..428,724

14 Claims; (Cl. 122-235) My discovery and invention relates in generalto the construction. and operation of fluidheating apparatus, and moreparticularly, to the construction and operation of steam. generatingunits in which they heat is supplied by the combustion of. a pulverizedslag-forming fuel.

In medium and high. capacity steam generating units of thetypedescribed, the. convection: heated. surface usually includes a bank ofrelatively closely spaced small diameter steam superheating tubesadapted to superheat the steamto a relatively high temperature.installations it is desirable to position a bank. of steamgeneratingtubes in the path of the heating gases from the furnacechamber to the superheater" tubesto screen the superheater from In. suchas low a value as possible to minimize the fan power requirements. For.this reasonit is cusradiation from the furnace chamber and reduce thegas temperature sufficiently that overheating of the super-heater tubesis avoidedwhile per-- mitting the desired high degree of superheat to beobtained. In. pulverized fuel fired installations of' this type, suchboiler screen tubesare designed to-serve the additional. function of reducing the temperature of slag particles in. a. molten or stickycondition suspended in the heating gases to a dry solid state ornon-adhesive condition to thereby minimize'the deposition of turnedgases can. mix with the higher temperature gases generatedby the burningfuel and thus reduce the average temperatureof the gas. stream leavingthe furnace chamber. Due'to therelatively low temperature at which gasescan be safely contacted with the operating parts of a fan or other gascirculating device, it has been necessary to withdraw the gases to berecirculated at a point. where their temperature has been reduced bycontact with a major portion of the convection heatedv surface of theunit. Gas recirculating arrangements of this type re-- suit in asubstantial increase in the fan power requirements of the unit and thuscorrespondingly increase the installation and operating costs of theunit.

It is widely recognized that in the designing .of convection heatedfluid heating apparatus, the

flow path of the heating gases should be arrangedfor a maximum contactbetween the heating gases and the heat absorbing surface whilemaintaining the draft loss through the unit at tomary to provide a flowpath: for the heating gases in. which flow influencing elements,- suchas baflle and wall arrangements, tending to produce local .eddy currentsin the'gas flow are eliminated as far as possible, so that a streamlinedstraight through flow. of the heating gases will takeplace.

Ihave. discovered howeverthat under certain circumstances factorsinducing substantial eddy currents in the gas flow; are not only notharmful, butin-fact can. behighly'beneficial in. providing an inherent.recirculation of a large portion of the heatinggases at a locationwhereby the average temperatureof the gases leavingthe furnace chamberwill. be reduced to a value at which safe wall temperatures for thesuperheater. tubes are insured'and, when the unit is flredwithaslag-forming fuel, the temperature of suspended slag particles. will bereduced below the fuelash softening temperature before contacting withthe superheater tubes. I have discovered that a gas recirculation.effect of the character described can be enhanced and maintained byaspecial construction of the bank of heat absorbing tubes firstcontacted by the heating gases-leaving the furnace chamber, further bycontrolling thedirection and velocity of the furnace gases adjacentv thelocation at which the recirculated gases. enter thefurnace chamber,andstill. further-by controlling the. path of travel of. the-heatinggases in the furnace chamber andapproaching the first bankof convectionheated tubes; I

In utilizing my'discovery; the high temperature heating gases leavingthe furnace chamber should be first directed across a bank of verticallyarranged. fluid. heatingtubes constructed and arrangedto have apredetermined relation of effective heat absorbing" area, tube height inthe gas flow passage; and gas pressure drop across the tube banktendingv to establish a thermal by providing ahigh velocity for theascending high temperature furnace gas stream adjacent the locationwhere the recirculated gasesreturn to-the furnace chamber, and directedat an angle.

away from the lower portion of the first convection heated bank. Suchhigh gas velocities will result in a lower static pressure condition atthis point, and thus an additional draft effect tending'to increase theflow of gas reversely across the lower portionof the tube bank. Ineffect the high velocity 'gas stream will-act as a fluid. jet to produceentrainment of the recirculated 7 ized fuel entering that section andmaintain the ture high velocity gas stream upwardly along the front walland roof in the'upper portion of thei horizontal cross-section in thelower portion below furnace chamber, so that the inertia effect of thesegases will naturally cause the hotter gases to concentrate along andflow across the upper portion of the first convection heated tube bank."

In the drawings: Fig. 1 is a sectional elevation of a pulverized fuelfired slag tap steam generating unit taken on the line of Fig. 2, andhaving a construction and arrangementof the furnace chamber andconvection heated fluid heatingsurfaceproviding a recirculation of asubstantial amount of heating gases to the furnace chamber;

Fig. 2 is a horizontalsection taken on 2-2 of Fig. 1; and 7 Fig. 3 is anenlarged view of a portion of the the line unit shown in Figs. 1 and 2diagrammatically il.ustrating the gas flow therein.

The steam generating unit illustrated in the drawings utilizes thevarious features of mydiscovery, and comprises a vertically elongatedfurnace chamber I0 defined by a-vertical front wall I I, oppositesidewalls l2, and a short rear wall I3, all of which are provided withvertical water desired high temperature conditions therein. A slagdischarge port 31 is providedin one of the side walls l2 at the floorlevel thereof through which molten slag depositing on the floor |4 maybe discharged either continuously or intermittently. The remaining.portions of the furnace wall tubes are left bare to provide a largeamount of radiant heat absorbing surface throughout the remainingheightof the furnace chamber.

With the described furnace wall construction the furnace chamber will beof substantial height and of rectangular horizontal cross-section, the

the rear v arch I6 being substantially greater than I the cross-sectionabove that arch. The vertical the furnace chamber which opens into a gasflow tubes connected into the steam generating circuit of the'unit. Thebottom of the furnace chamberis closed by a floor I4 and the top by arearwardly inclined roof I5. The vertical'rear wall l 3sextends alongonly the lower part of thefurnace chamber, and is continued as aforwardly inclined "steam and water drum 25,'1ocated above the roof.Each of the side walls I2 of the furnace chamber is cooled by a row ofwater tubes 25 connected at their lower ends to a longitudinal header2'! supplied by the drum 2| and at their upper ends to a header28discharging to the drum 251% A I row of tubes 30 extends from the drum2| along the remaining portion of the floor I4 and thence upwardly alongthe rear wall I3, arch I6, and vertical wall section I! to a smalldiameter transverse drum -3| invertical alignment with the drums 2| and25. Downcomerpipes 32 externally connect the bottom of the drum 25 tothe Upper and lower rows of pulverized coal burner nozzles 35 aredownwardly inclined and arranged to discharge between the portions ofthe tubes 20 at the lower part of the front wall I I towardsthe rearwall I3. 'Combustionair is supplied through a windbox 36 surrounding theburner nozzles. The portions of the remaining furnace wall tubes fromthe floor level thereof to'a; level above the burner level are coveredwith metallic studs and refractory to promote combustion of thepulverpassage 40 of uniform width between the side walls l2. The passage40 is further defined by a fiat roof 4| forming a continuation of theroof l5 and a rearwardly inclined floor 42 having a short verticalsection 43 at its forward end connecting with the top of therear wallsection I'I, so that the passage 4|i progres-sively decreases in heightrearwardly of the unit. :m the passage 40 is 'arranged the, convectionheated steam generating and superheating surface of the unit consistingof a bank of vertical steam generating tubes 44 and a. three-sectionpendant type superheater 45. The steam generating tubes 44 are connectedat their-lower ends to the transverse header 3| and have their upperportions extending through the roof |5.and connected to the steam andwater drum 25. The 'rearmost section ofthe superheater 45 is connectedto the steam space of the.

' tion, passage 40; below the superheater is lined with block-coveredwater tubes 42"- having their lower ends connected to the drum 3| andtheir upper ends to a transverse header 41. As shown in Fig. 2, thesuperheater passage 51 does not extend the full width of the unit but isseparated by a water cooled bailie 48extending parallel to the sidewallls I2 fromthe front end of the s uperheater to the rear end of theunit, to provide a superheater by-pass passage 49- Rows of steamgenerating tubes. 50'are positioned in the by-pass fpassage with theirlower ends connectedto the header 41 and their upper ends connected toan upper header and the steam and water drum 25. V The remaining portionof the gas flow path is.

formed by a vertical passage 5| extending downwardly at the rear end ofthe unit and occupied by a plurality of vertically spacedseriallyconnected groups of horizontally arranged'multiple looped transverseeconomizertubes 52. The bailie 48 has an extension in the passage 5| toprovide a continuation of the superheater andby-pass passages.

across the economizer tubes,and dampers 53 are positioned 'therebelow toregulate the new of the heating gases through these passages.- The gaseson leaving the economizer tubes turnfabove an ash hopper 54andflow-upwardly through a tubular air heater 55 and induced draft fanto'a suitable stack connection.. j a I I .1 Steam generating units ofthe general character described are usually designed to operate in thefollowing manner:

Pulverized fuel and combustion air are intro- -duced through the lowerportion of the front wall H and the pulverized fuel burned in suspensionin the lower part of the furnace chamber Hi, the supply of combustionelements and-heat absorbing surface in the lower section of the furnacechamber being proportioned to maintain a normal mean temperaturetherein, particularly adjacent the floor l4, above the fuel ash fusiontemperature, whereby the burning fuel will'be deposited on the floor 14as'molten slag and withdrawn through the slag outlet 31. Thegaseous'products of combustion and any ash particles in suspensiontherein are designed to pass upwardly through the furnace chamber, andare cooled during'their passage by the radiant heat absorption of thefurnace wall tubes. Due to the furnace draft efiect'the high temperatureheating gases and suspended ash particles move laterally in the upperpart of the furnace into the gas passage 40. The temperature of thegases and suspended ash particles is re- 44 to a value designedly belowthe fuel ash fusion the ash particles separating from a 6 tion of theheating gases entering the tube bank One of these features is embodiedin-thesteam generator illustrated, i. e., theconstruction andproportioning of thefirst bank of convection heated tubes in the gaspass.40, and particularly, the relative proportioning of the effective-heatabsorbing area. thereof, tube height, andgas pressure drop'or draft lossthereacross. Bywayof example and notof limitation, the tube bank 44 isshown as composed of 95 tubes 4" O. D. arranged in flve'rows spaced 12"apart. The tubes in the two rearmost rows are spaced on 9'5 centerswhile the tubes in the forward rows are on 18", centers. The rearmostrow is spacedji' from the adjacent row of superheater tubes. Theeffective length of the tubes in the bank, 1. e.,"from' the drum 3| tothe superjacent roofportion is 29'6", the width ,duced by their flowacross the bank of screen tubes of the furnace chamber in this planebeing 2011. The expected performance of thisunitoperating at a'capacityof 345,300 lbs. of steam per hr. with a straight through flow and norecirculation of temperature before they contactwith the more closelyspaced superheater tubes'in the superheater passage 41. With pulverizedfuel-having the designed ash fusion temperature characteristics, the ashparticles should be in a dry condislagging problems in the superheater.Any ash depositing on' the superheater tubes can therefore be blown offand down the inclined floor 42 to the furnace chamber. The gases flowingthrough the superheater passage'il or by-pass passage 49, or both, flowdownwardly across the economizer tube banks 52 and thence upwardlythrough the tubular air heater 55 to the induced draft fan and stackconnection. Under proper operating conditions therefore the ash in thefuel separated during combustion or during the subsequent pas sageof theheating gases will be removed as a molten slag through the slag outlet31 at the furnace bottom or as a dry ash through the ash hopper'54, andthe heating gases will flow through the gas passage 40 in a single passflow over the tube bank 44, superheater tubes 45, economizer tubes 52and air heater 55 successively. I When, however, pulverized fuel havinga lower fuel ash fusion temperature than thatfor which the unit isdesigned is used, or when the rate of heat abculating a substantialportion of the heating gases leaving the furnace chamber after theirtemperature has been reduced by their passage across the boiler slagscreen tubes 44,- without requiring special circulating fans or othermechanicaldevices for this purpose and without materially adding to thefan power requirements of the unit. The gas recirculation effect, andconsequently the proportion of the gases recirculated, I have found tobe dependent upon several features which Jointly contribute to therecirculation of a large proporcharacter described without resultgasestothe furnace chamber would have a. gas temperature leaving the'furnacechamber of 2320 F. and leaving the tube bank 44 of 2036 F. Under suchconditions the gas pressure drop across the tube bank would beapproximately .025 in. H2O.

tion on reaching the superheater and present no Thus an appreciabletemperature reduction in the heating gases flowing across the tube bankwould .40 I differential to exist.

occur, yet the draft loss would be of a low order. With a draft loss andheat absorption of this 'order, the temperature difference of theases'at opposite sides of the tube bank 44in conjunction with thesubstantial height of the tube bank produces a draft differential at anelevation near the a lower end of the tubebank sufficiently great toestablish and maintain a gas flow forwardly through the tube bank atthis position. The forward flow, or recirculation, of the gases willcon- .tinue as long as there is a tendency for this draft The describedthermal siphon action thus effects a return-of some of the gases afterbeing cooled by their'passage'through the tube bank 44;

Another feature of the illustrated construction which substantiallypromotes the gas recirculation-effect is the relative arrangement of thefuel burners 35 and rear furnace arch or roof I6, As diagrammaticallyillustrated in' Fig. 3, the fuel burners discharge downwardly andrearwardly. towardsthe'rear wall l3 so that the furnace gases generatedwill sweep upwardly along the rear wall and be directed forwardly andupwardly by the forwardly. inclined arch I6 and at a relatively high'velocity when leaving the forward end of the arch. A correspondingly lowstatic pressure condition will thus be created in this area which willcause an additional draft effect on the gases fiowingforwardly acrossthe lower part of the tube bank 44. The ascending furnace gas streamthus acts as. a fluid jet to produce entrainment of the recirculatinggas flowing forwardly through the tube bank, as indicatedin Fig. 3. Amixing of the lower temperature recirculated gases with the ascendinghigh temperature gases takes place, lowering the average temperature ofthe gases entering the furnace side of the tube bank. I The gasrecirculation effect is also aided by the direction given to the gaseson leaving the rear arch l6, 1. e., upwardly along the upper part of thefront wall II and roof [5, The inertia of aaiaosa 7 g that these gases.naturally tend to enter the unper part of the tubejbank." I y and atotal steam temperature of 900 F. High:

ve'locity thermocouple measurements :of .gas "tem-.

peraturestaken at different points around the tube bank 44' while theunit was-"operating at a capacity of approximately 350,000. lbs: perlhr. are

indicated on Fig. 3. The direction of gas .flowat different points inthis area was also 'deter- I mined by test. There was found :toi'be avery substantial recirculation .of the gases, as 'indi- 'cated bythe'arrowsinlFig, 3. Bycalculating the temperature drop through theupper portion of the tube bank .over which the gases were found to beflowing rearwardly with various weights of flue gas, it was .estimatedfrom observations and calculations that approximately 100% of thefurnace gases generated were being recircu lated at this load,Anadditionaliquantity of gas partly enters the tube .ban'k. and-then.returns to the furnace chamber with the recirculatedgases,

in front of the lower end ofthe tube bank relative to that at the lowerrear part thereof. v 1 The described gas recirculation is highly belie--ficial in regard to ash and slag accumulations.

The rearwall I 3, floor 14 and rear part of the lower side walls haveslag deposited thereon in a fluid state. The slag fluid'zon'e extends tothe chamber to bedeflacted at a relatively, high velocitypast ,and in adirection away from the- '25' accounting for the higher gas temperaturevalues per .part of'the furnace chamber, and causing the gaseousproductsof combustion vflowing upwardly through the lower part of the furnacechamber to be deflected mainly. along the side of said furnace chamberopposite saidvtu'be bank.

g .2. Themethod of burningya fuel which c0m-.

prises introducing the fuel and combustion air into a furnace chamberand burning the same in the lower part of saidchamber, directing thegaseous products of combustion in the upper part of the furnace chamberintovcontactiwith a vertically'arranged bank of fluidheating tubes inthe Y fumace chamber outlet and ofa height, draft I close and heatabsorbing capacity "proportioned to establish and maintain a thermalsiphon effectrproviding a circulation of a portion of the gaseousproducts of combustionin the furnace chamber outlet in a reversedirection-across the lower :portion'of said tube bank and into the upperpart of-the furnace chamber, and causing the gaseous products ofcombustion flowing upwardly through the lower part of the furnace lowerportion of the {tube bank toypromote the prises introducing a'stream ofthe fuel and combustion air into a vertically elongated furnace chamberand burning the same in suspension in front end of the arch is, asindicated in broken lines in Fig. 3, but the slag .fluid zonesharplyends .at the lower end of thevertical wall portion l7. Therecirculated gases thus cause thesuspended slag particles to passrapidly through the I the lower part of said chamber, directing thegaseous productsof combustion in the upper part of thefurnace chamberinto contact witha vertically arranged bank of fluid heating tubes inthe yfurnace chamber out-let. and of a height, draft sticky stage beforereaching the convection heated surface. 'I'hatbare tube surface ismaincumulations and that only dry ash deposited on the sloping floorsbelow the'super'heater and over the header 3 I, from which it was sweptback into.

the furnace chamberby the gases. 1

Fluid heating apparatus utilzing the described: features of my discoverywill thus have an effective use ofthe furnace chamber volume and a meanheating gas temperaturefat the furnace: chamber outlet which will avoidoverheating of burning the same in suspension inthe lowerpa'rt' of saidchamber, directing the gaseous products of combustion in theupper partof thefurnace. chamber laterally into contact with a vertically arrangedbank of fluid heating tubes in the furnace chamber outlet and of asubstantial height; and sufl'iciently low draft loss and high heatabsorbing capacity to establish and maintain at substantially themaximum designed continuous rate of operation a thermal siphon effectproviding a circulation of a substantial portion of the aseous productsof combustion in the furnace chamber outlet in a reverse directionacross the 4 tained' clean by the downward sweep of the coolerjrecirculated gases. It was-also found that the lossv and [heatabsorbing capacity proportioned to establish and maintain athermalsiphoneffect 1 providing a circulation of a portionof the gaseous products ofcombustion .in the furnace chamber outlet in a reverse direction acrossthe lower portion of saidtube bank and into the upper tube bank 44 wasmaintained clean of slag acpart of the furnace chambenand directing thepart of the furnace chamber at a relatively high velocity and at anangle to the direction oi re-,

- circulated gas flow across the lower partof said tube bank andrev'ersely of the direction of entrance of the fuel and air stream topromote the' gas recirculation effect and mixing of. the recirculatedgases.

'4. The method of burning a fuel which comprises introducing the fueland combustion air into a vertically elongated; furnace chamber andburning the same in suspensionin the lower'part of saidv chamber;directingthe' gaseous products of combustion in the upper part :of'thefurnace chamber into contact with a vertically arranged bank of fluidheating tubes of a substantial height andsufiiciently low draft loss andhigh heat ab sorbing capacity to establish and maintain a thermal siphonefie'ct'providin'g a circulation of a substantial portion ;of'thegaseous products of combustion in-a reverse direction across thelower portion of said tube bank'and'i-nto'the upper part ofthe furnacechamber, and directin'gthe gaseous products of combustion leaving thelower part of the 'furnace chamber at a relatively high .velocity'and ina direction adjacent the lower portion of said tube bank reversely ofthe entering fueland combustion air and of "the gas flow across theupper part of said tube hank to promote the gas recirculation effect andmixing of the recirculated gases.

lower portion of said tube bank and into the up- 5. The method ofburning a pulverized slag,-

forming fuel which comprises'introducing the fuel and combustion airinto a furnace chamber and burning the same in suspension in the lowerpart of said chamber under a normal mean temperature therein above thefuel ash fusion temperature, removing molten slag depositing in thebottom of the furnace chamber, and directing the gaseous products ofcombustion in the upper part'of the furnace chamber laterally intocontact with a vertically arranged bank of fluid heating tubes in thefurnace chamber outlet and of a height, draft loss and heatabsorbingcapacity proportioned to establish and maintain a thermal siphon effectproviding a circulation of a major portion of the gaseous products ofcombustion at the maximum continuous designed capacity in thefurnacechamber outlet in a reverse direction-across the lower portion ofsaid tube bank and into the upper part of the furnace chamber sufficientto reduce the average gas temperature at the furnace chamber outletbelow the fuel ash fusion temperature. v

6. The method of burning a pulverized slagforming fuel which comprisesintroducing a stream of the fuel and combustion air into a verticallyelongated furnace chamber and burning the same in suspension in thelower part of and burning the same in suspension in the lower part ofsaid chamber under a normal mean temperature therein above the fuel ashfusion tern perature, removing molten slag depositing in the bottom ofthe furnace chamber, directing the gaseous products of combustion in theupper part of the furnace chamber laterally into contact with avertically arranged bank of fluid heating tubes in the furnace chamberoutlet and having a substantialheight and high heat absorbingcapacityproportioned relative to draft loss therethrough' to establish andmaintain a circulation of a portion of the gaseous products ofcombustionin the furnace chamber outlet in a reverse direction acrossthe lower portion of said tube bani: and into the upper part of thefurnace chamber, and causing the gaseousproducts of combustion flow ingupwardly through the lower part of the funnace chamber to be deflectedat a relatively high velocity and in a direction away from the lowerportion of said tube bank tending to promote the gas recirculationeffect and mixing of he recirculated gases.

' 9. The method of burning a pulver zed slagforming fuel which comprisesintroducing the fuel and combustion air into a vertically elongated saidchamber, directing the gaseous products of combustion in the upper partof thefurnace chamber laterally into contact with a vertically arrangedbank of fluid heating tubes in the furnace chamber outlet and of asubstantial height, low draft loss and heat absorbingcapacityproportioned to establish and maintain a circulation of a substantialportion of the gaseous products of combustion in the furnace chamberoutlet in a reverse direction across the lower portion of said tube bankand into the upper part of the furnace chamber, and directing-thegaseous products of combustion leaving the lower part ofthe furnacechamber past the lower portion of said tube bank opposite to thedirectionof the entering stream of fuel and combus'tion'air to promotethe gas recirculation'eifect and mixing of the recirculated gases. I

'7. The method of burning a pulverized slagforming fuel which comprisesintroducing the fuel and combustion air into a vertically elongatedfurnace chamber and burning the same in suspension in the lower part ofsaid chamber under a normal mean temperature therein above the fuel ashfusion temperature, removing molten slag depositing in the bottom of thefurnace chamber, directing the gaseous products of combustion in theupper part of the furnace chamber laterally into contact with avertically arranged bank of fluid heating tubes in the furnace chamberoutlet and of a substantial height and sufficiently low draft loss andhigh heat absorbing capacity to establish and maintain a thermal siphoneffect providing a circulation of a portion of the gaseous products ofcombustion in the furnace chamber outlet in a reverse direction acrossthe lower portion of said tube bank and into the upper part of thefurnace chamber sufficient to reduce the average gas temperature at thefurnace chamber outlet below the fuel ash fusion temperature, andcausing the gaseous products of combustion flowing upwardly through thelower part of the furnace chamber to be deflected towards and along theside of said furnace chamber opposite said tube bank.

8. The method of burning a pulverized slagforming fuel which comprisesintroducing thev fuel and combustion air into a furnace chamber furnacechamber and burning the same in suspension in the lower part of saidchamber and under a normal mean temperature therein above the fuel ashfusion temperature, removing molten slag depos ting in the bottom of thefurnace chamber, directing the gaseous products of combustion in theupper part of the furnace chamber laterally into contact with avertically arranged bank of fluid heatingtubes in the furnace chamberoutlet and of a substantial height and sumciently low draft loss andhigh heat absorbing capacity to establish and mainta n a thermal siphonefiectproviding a circulation'of a substantial portion'ofthe gaseousproducts of combustion inthe furnace chamber outlet in a reversedirection across the lower portion of said tube bank, and into the upperpart of the furnacev chamber suflicient to reduce the averagegas'temperature at the furnace chamber outlet below the fuel ashsoftening temperature, and directing the gaseous products of combustionleaving the lower part of the furnace chamber at a relatively highvelocity and in asdirection adjacent the lower portion of said tu e bankreversely of the direction of the entering stream of fuel and combustionair to prom'ote the gas recirculation effect and mixing of therecirculated gases.

, 10. In a steam generating unit having a vertically elongated furnacechamber and a heating gas outlet at one side of the upper portionthereof,

a lateral gas passage opening to said gas outlet, a vertically arrangedbank of steam generating tubes in said gas passage having a substantialheight, low pressure drop and high heat absorbing capacity, and a groupof closely spaced steam superheater tubes in said gas passage spacedfrom said tube bank, the method of burning fuel in.

said unit which comprises burning the fuel in the lower part of saidfurnace chamber, d recting the furnace gases in the upper part of thefurnace chamber across the upper part of said tube bank, and causing thefurnace gases flowing upwardly through the lower part of the furnacechamber to be deflected in a diverging direction adjacent the lowerportion of said tube bank tending to promote a flow of heating gasesdownwardly between said tube bank and superheater tubes and reverselyacross the lower portion of said tube bank into the furnace chamber.

' furnace gases flowing upwardly throughthe lower 11. In a steamgenerating unit havin a vet-1 tically elongated furnace chamber and aheating gas outlet at the rear side of the upper portion thereof, alateral gas passageopening to said gas outlet, a vertically arrangedbank of steamgenerating tubes in said gas'passage having a substantialheight, low pressure'drop and high} heat'absorbingcapacity, and a groupof closely spaced steam superheater tubes in said gas passage spacedrearwardly of saidtube bank, the

, methodof burning a pulverized slag-forming fuel insaid'unit whichcomprises burning the fuel" in suspension in the lower part of saidfurnace chamber under 'a normal mean temperature therein above the fuelashfusion temperature;

removing molten slag depositing in the bottom of the furnacechambendirecting the furnace gases in the upper part of the furnacechamber across part of a vertically, elongated furnace chamber having'agas outletin one side of they upper partv thereof, directing thegaseous-productsof come bustion in the upper part of the furnacecham-'ber laterally into contact with a vertically arranged bank of. fluidheating tubes across the furnace chamber gaswoutlet and of a height, I

' draft loss andheat absorbingcapacity proporthe upperpart of said tubebank, and causing the part of-the furnace chamber to be deflected in adirectiontending to promote a flow of heating gases downwardly betweensaid tube bank and superheater tubes and reversely across the lowerportion ofsaid tube bank into the furnace chama ber to an extentsufiioient to reduce'the average tioned to establish and maintain athermal siphon effect providing a circulation of a portionof the gaseousproducts ofcombustion in the furnace chamber outlet in a reversedirection, across the" lower portion of said tube bank and into theupper part of the furnace chamber,- and causing the gaseous products ofcombustion flowing up.- wardly through the lower part of they furnacechamber -tobe deflected atv a converging angle to the direction. ofrecirculated gas flow across the lower part of said tube bank,to'promotethe gas recirculation effect-andmixing of the recirculatedgases.

14.-The method of burning a fluid fuel con- .taining an incombustibleashwhich comprises in.-

temperature of the heating gases contacting withj said superheater tubesbelow the fuel ash soften-1 ingtemperature;

12..In a steam generating unit-having a vertically elongated furnacechamber and a heating; gas outlet at the rear side of the upper portionthereof, a lateralgas passage opening to said gas outlet, 'a verticallyarranged bank of steam gen-l erating tubes in said gas passage having asub-1 stantial height, low pressure drop and high heat absorbingcapacity, and a group of closely spaced steam superheater tubes in saidgas passage spaced rearwardly ofsaid tube bank, the method of buming apulverized slag-forming fuel in said unit which comprises burningthe-fuel in suspen-; sion inthe lower part of said furnace chamber,

under a normal mean t emperatu're therein. above the fuel'ash fusiontemperature, removing molten slag depositing in the bottom of thefurnace chamber, directing the furnace gases in the upper part'of thefurnace chamber across the upper part of'saidtube bank, and directingthe furnace;

gases leaving the lower part of the furnace chamber at a relatively highvelocity and in a direction reversely of the direction of the enteringstream 'of fuel and combustion air to promote a flow of heating gasesdownwardly betweensaid tube bank and superheater tubes and reverselyacross the lower portion of "said tube bank into the furnace chamber toan extent suflicient to; reduce the average temperature of the heatinggases contacting with. said superheater tubes belowthe fuel ashsoftening temperature.

13. The method of burning fuel which com-v prises introducing andburning a fuel in the lower troducing andburning a fuelin the lowerpart. of a vertically elongated furnace chamber having a gas outlet inone side of the upper part thereof, removing (molten ash from the bottomof. the

furnace chamber, directingthe gaseous productsof combustion in the upperpart ofthe furnace i chamber laterally into contact with'a verticallynace chamber to be deflected at a. converging angle to the direction ofrecirculated gas flow across the lower part of said tube bank to promotethe gas recirculation effect and mixing of the recirculated gases. I I,1 I 1 v PAUL R (EROSSIVLAN. REFERENCES men a The following referencesarev of record in the flle'of this patent:

' UNITED STATES PA'rEn'rs Number Name I Date 2,231,872 Bailey 8'; 8 ,1Feb. .18, 1941 2,254,226 K0611 Sept. 2, 1941 2,252,061 Cassidy Aug, 12,1941 2,263,433 Allah NOV. 16, 19 41 Certificate of Correction Patent No.2,416,053. February 18, 1947;

PAUL R. GROSSMAN It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows: Column 9, line 42, claim 6, before the word opposite insertin a direction; and that the said Letters Patent should be read withthis correction thereinthat the same may conform to the record of thecase in the Patent Ofiice.

Signed and sealed this 28th day of September, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommissioner of Patents.

